In today’s Academic Minute, Dr. Larry Widrow of Queen’s University reveals evidence for a galactic shakeup, right in our cosmic backyard.

Larry Widrow is a professor of astronomy at Queen’s University in Kingston, Ontario. His research interests include theoretical cosmology, dark matter, and the structure, dynamics, and evolution of galaxies. Widrow, together with his collaborators Susan Gardner, Brian Yanny, Scott Dodelson, and Hsin-Yu Chen, reported their recent finding in the Astrophysical Journal. His work has appeared in a number of additional peer-reviewed publications and he holds a Ph.D. from the University of Chicago.

The most prominent component of the Milky Way Galaxy is its disk, which is composed of some 50 billion stars along with a fair bit of gas and dust. The disk rotates, which keeps it from collapsing under its own self-gravity. A typical star in the disk orbits the Galactic center on an approximate circular path, but also moves perpendicular to the Galactic plane.

One such star is our Sun, which orbits the center of the galaxy at roughly 230 kilometers per second or about 500 thousand miles per hour. When viewed from the Sun's frame of reference, nearby stars appear to move in random directions. In 1932 the Dutch astronomer, Jan Oort, used observations of these motions to construct one of the first physical models of the Galaxy and to attempt to infer the amount of mass in the Galactic disk.

Following in his footsteps, my colleagues and I used recent data from the Sloan Digital Sky Survey to see if we could learn something about dark matter, the mysterious, non-luminous material thought to keep our galaxy from flying apart. To our surprise, we found significant differences between the distribution of stars to the North and South of the Galactic midplane, something Oort and others had not noticed.

It is as if a wavelike perturbation had swept through the Galactic disk and displaced stars from their equilibrium orbits. Such a perturbation may have been created by a satellite galaxy. Our galaxy has over two dozen visible satellites and an unknown number of non-luminous dark matter satellites. If one were to pass through the Galactic disk near the position of the Sun, its gravitational field could set off waves much like the ripples in a pond when a stone is thrown in. Thus, the motions of stars neighboring our sun may hold clues of a past cosmic encounter.